day one noun 2adverb 1forms 1 noun2 •The very beginning.I've been with the project since day one. •(US, slang) A close, longtime friend, one who has been around since the beginning.Raise a cup up for all my day ones; two middle fingers for the haters […] — 2017 adverb1 •At the very beginning; when something is entirely new. formsday ones
